Digital radiography (DR) is increasingly accepted as an alternative to film-based imaging technologies that rely on photosensitive film layers to capture radiation exposure and thus to produce and store an image of a subject's internal physical features. With digital radiography, the radiation image exposures captured on radiation-sensitive layers in X-ray imaging panels are converted to electronic image data which is then stored in memory circuitry for subsequent read-out and display on suitable electronic image display devices.
Generally, a scintillator (or scintillation) screen responds to incident X-ray radiation by generating visible light that is, in turn, detected by a photodetector having photosensors. The light information from the photodetector is subsequently transmitted to charge amplifiers. The outputs of the charge amplifiers are then typically applied to other circuitry that generates digitized image data that then can be stored and suitably image-processed as needed for subsequent storage and display. However, because scintillator materials respond to incident x-ray radiation by emitting light over a broad range of angles, there is some inherent amount of scattering in the detection process. This reduces the optical efficiency of image formation due to loss of light, signal crosstalk, and related effects, and tends to degrade image quality.
For example, a scintillator screen typically has a scintillation layer formed on a support that is highly transmissive to incident X-ray radiation. A protective overcoat layer may optionally be provided over the scintillation layer. Scintillator material in the scintillation layer responds to incident X-rays by emitting photons toward a photosensor, but over a broad range of angles, including angles at which the emitted light is effectively wasted due to total internal reflection (TIR) effects within the scintillation layer or, if provided, the overcoat layer. But so long as there is good optical coupling between the scintillator screen and the photodetector, a sufficient amount of the emitted signal is directed toward the photosensor.
In practice, there is often poor optical coupling between the scintillator screen and the photodetector. Air gaps or airborne contaminants, such as dust, can be trapped between the scintillator screen and the photodetector. For light at very small angles of incidence (relative to normal), the net effect of air gaps or airborne contaminants can be negligible. But for light at larger angles, air gaps or airborne contaminants can cause problems. When light is incident from a dense medium with a higher index of refraction, n, to a rare medium with a lower index of refraction, n′, (e.g., n′=1.0 for air), TIR may occur at the interface of the two media depending on the angle of incidence. This means that some portion of light is lost, and another portion can be redirected to the wrong photodetector, i.e., crosstalk. The net effect includes lost efficiency and reduced spatial resolution, which is generally measured by the modulation transfer function (MTF). MTF is widely used in many imaging applications as a quantitative way of determining or measuring the resolution or sharpness of imaging devices. In digital radiography, MTF is dominantly decided by the scintillator screens used for X-ray absorption. Therefore, poor optical coupling due to the presence of air gaps or airborne contaminants at the interface of the scintillator screen and the photodetector can lead to increased TIR, reduced MTF, and result in poor image quality.
Conversely, improved optical coupling between the scintillator screen and the photodetector would help to boost efficiency and improve overall image quality accordingly. However, previously proposed solutions have shown only limited success, or may achieve improved optical coupling at the cost of increased complexity and higher expense, or may inadvertently introduce other problems. For example, while conventional pressure sensitive adhesives (PSAs), such as acrylic-based adhesives and laminates, have been used in the past to couple scintillator screens and photodetectors, PSAs are aggressively tacky at room temperature and strongly attract airborne contaminants such as dust. As such, when PSAs are used, extreme care must be taken to avoid trapping dust particles in the adhesive. For example, the scintillator screen should be stored and adhered to surfaces in a clean environment to minimize the introduction of contaminants.
Thus, while prior techniques may have achieved certain degrees of success in their particular applications, there is still room for improvement. Solutions that reduce or eliminate air gaps and/or airborne contaminants at the scintillator screen/photodetector interface without an elaborate number of steps and using materials appropriate for the scintillator or detector components would be particularly helpful.